1. Field of the Invention
The invention relates to an organic electroluminescent device, and more particularly to an organic electroluminescent device having improved light-emission characteristic.
2. Description of the Related Art
An organic electroluminescent device (hereinafter, referred to simply as “organic EL device”) is a light-emitting device under the principle that fluorescent material performs light emission by re-combination energy generated when holes introduced through an anode and electrons introduced through a cathode are re-combined, which re-combination is caused by applying an electric field to the organic EL device.
After the organic EL device driven at a low voltage and comprised of a multi-layered structure has been reported by C. W. Tang and S. A. VanSlyke in Applied Physics Letters, Vol. 51, pp. 913, 1987, various studies have been made about an organic EL device composed of organic material. The organic EL device suggested by C. W. Tang et al. includes a light-emitting layer composed of tris-(8-hydroxyquinolinol aluminum), and a hole-transporting layer composed of triphenyl di-amine. The multi-layered structure provides advantages that holes can be injected into a light-emitting layer with a higher efficiency, that electrons injected through a cathode can be blocked to thereby enhance a generation efficiency of excitons generated by re-combination, and that excitons generated in a light-emitting layer can be shut in the light-emitting layer.
An organic EL device generally has either a two-layered structure including a hole transporting layer and an electron transporting light-emitting layer, or a three-layered structure including a hole transporting layer, a light-emitting layer, and an electron transporting layer. Various attempts have been made about a device structure and a method of fabrication in such a multi-layered structure device in order to enhance an efficiency of re-combination of injected holes and electrons.
As suggested in Japanese Unexamined Patent Publications Nos. 8-20771, 8-40995, 8-40997, 8-259935, 8-543397, and 8-87122, triphenyl amine derivative and aromatic diamine derivative of star-burst molecules such as 4, 4′4″-tris(3-methylphenylphenylamino)triphenylamine, and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine, are well known as materials of which a hole transporting layer is composed.
As materials of which an electron transporting layer is composed, oxadiazole derivative, triazole derivative and so on are well known.
As materials of which a light-emitting layer is composed, chelate complex such as (8-quiolinolate)aluminum complex, coumarin derivative, tetraphenylbutadiene derivative, bisstyrylarylene derivative, and oxadiazole derivative are well known. It has been also reported that those materials emit a light in a visible range including blue to red lights. For instance, those materials are used for color display device, as suggested in Japanese Unexamined Patent Publications 8-239655, 7-138561, and 3-200289.
An organic EL device is free from a limitation that an alternative current and a high voltage have to be applied thereto, unlike an inorganic EL device. In addition, it is considered that an organic EL device could provide many colors due to variety in organic compounds, and hence, an organic EL device is expected to apply to a full-color display.
In application of an organic EL device to a full-color display, it is necessary to have light-emission of three major colors, that is, red, green and blue. Many examples have been reported about green light-emission. For instance, there has been suggested a device including aluminum complex of 8-quinolinol Applied Physics Letters, Vol. 51, pp. 913, 1987), and a device including diarylamine derivative (Japanese Unexamined Patent Publication No. 8-53397).
There have been reported many examples of blue light emitting devices. For instance, Japanese Unexamined Patent Publications Nos. 5-295359, 7-53955, and 6-132080 have suggested a device including stilbene family compound, a device including triarylamine derivative, and a device including styrylized biphenyl compound, respectively.
Japanese Unexamined Patent Publication No. 8-245955 has suggested an organic EL device in which various light-emitting materials emitting various colored lights may be selected by using an electron transporting material having stability as a thin film, resulting in reduction in brightness.
However, there are quite few examples about an organic EL device providing red light emission. For instance, Japanese Unexamined Patent Publication 3-152897 has suggested a method of converting a wavelength of a blue light in a fluorescent pigment layer. Japanese Unexamined Patent Publications Nos. 7-272854, 7-288184, and 8-286033 have suggested red light emission accomplished by doping red fluorescent pigment into a green or blue light emitting layer. Examples of an organic EL device providing red light emission are few relative to examples of an organic EL device providing green or blue emission.
Furthermore, the above-mentioned organic EL devices providing red light emission cannot provide sufficient brightness, color purity, efficiency, and lifetime, and hence, need to be improved.
Apart from the Publications listed above, Japanese Unexamined Patent Publication No. 3-791 has suggested an electric field light-emitting device including a fluorescent light-emitting layer containing perylene compound.
Japanese Unexamined Patent Publication No. 4-6795 has suggested an organic electroluminescent device including an electrode through which electrons are injected and which has at least two-layered structure.
Japanese Unexamined Patent Publication No. 4-334894 has suggested an electric field light-emitting device including an organic thin film. The suggested electric field light-emitting device includes a layer composed of a hole transporting light-emitting material, and a layer composed of both a hole transporting light-emitting material and an electron transporting material.